Damped electro-acoustic high frequency transducer



Dec. 2, 1969 w. HECHT 3,482,062

DAMPED ELECTRO-ACOUSTIC HIGH FREQUENCY TRANSDUCER Filed April 18, 196720 Q I 9 fizz-a" gzzzzesz g J k\\\\\\\\\\\\\\ INVENTOR. HECHT ATTORNEYSWILLIAM United States Patent 3,482,062 DAMPED ELECTRO-ACOUSTIC HIGHFREQUENCY TRANSDUCER William Hecht, 80 Norfolk St., Bergenfield, NJ.07621 Filed Apr. 18, 1967, Ser. No. 631,732 Int. Cl. H04r 9/00, 9/06,9/10 U.S. Cl. 179-115.5 6 Claims ABSTRACT OF THE DISCLOSURE A dampedelectro-acoustic high frequency transducer which provides a chamberfilled with a viscous substance which is relatively incompressible,which chamber is defined by the inner wall of a voice coil form, the endof a pole piece, and an elastomeric cap affixed to the voice coil form;movement of the voice coil form with respect to the pole piece variesthe volumetric capacity of the chamber, causing the viscous material tomove the elastic cap and exert thereby a damping effect upon theexcursions of the voice coil form, and the diaphragm connected to it.

In the reproduction of musical sounds with a high degree of fidelity,loud speaker systems usually utilize a variety of transducers withappropriate crossover networks. It is common practice to use, inaddition to a low frequency transducer, a mid-range transducer forreproducing the frequencies from 200 to 2,000 Hz. and a high rangetransducer for reproducing the frequencies from the vicinity of 1,000 to20,000 Hz. Such transducers have a diaphragm, which may be of variousshapes, but which is usually conical, made of an air impenetrablematerial and, consequently, capable of moving air. The diaphragm isattached to a voice coil form and the voice coil form is disposed withrespect to a magnet to which it moves axially. Since the diaphragm ismade of somewhat resilient material, it will assume a normal position.Signals applied to the voice coil wound on the form will cause it tomove with respect to the magnet and to operate the diaphragm incorrespondence with the signal. By reason of the natural resonance ofthe transducer, it is normal for such transducers to resonate in thearea of 200 to 2,000 Hz. This introduced distortions into the soundsreproduced which were not present in the original signal. The idealtransducer is one which gives a flat response with respect to the signalapplied to the voice coil.

In order to achieve such a response, efforts are made to dampen theresponse sufficiently to nullify the natural resonance of thetransducer. Such efforts frequently require substantial modification ofthe transducer system which adds to the complexity, invites additionalwear and breakdown, and may introduce other additional resonances anddistortions. It has been found that it is possible to make a very simpleaddition to a transducer that will have the effect of establishing asubstantially linear relationship between the reproduced and the appliedsignal. This is accomplished by forming a chamber, the side wall ofwhich is defined by the inner wall of the voice coil form, one end wallby the pole piece, and the other end wall by an elastomeric cap aifixedto the other end of the voice coil form. In this chamber, a relativelythick viscous material is positioned. Thus, when the voice coil formmoves,the volumetric capacity of the chamber is either increased ordecreased. Since the viscous material in the chamber is relativelyincompressible, the movement of the voice coil form will cause thisviscous material to either distend or suck in the elastomeric materialthat caps the chamber. The force required to move the elastomeric cap issufficient to exert a damping effect upon the excursions of thediaphragm and, in this manner, produces a linear response.

This object and advantage may be attained, as well as other objects andadvantages, by the device shown by way of illustration in the drawingsin which FIGURE 1 is a vertical, sectional view of the transducer.

Referring now to the drawings in detail, there is provlded a pole plate9, a pole piece 10, a yoke 11, and a magnet 12. A dust cap 13 isattached to the pole plate 9 and surrounds the magnet 12. A voice coilform 14 surrounds the pole piece 10 and has its internal diameterdisposed extremely closely to the pole piece 12 without coming intocontact with it. A voice coil 7 is wound on the voice coil form 14; thevoice coil form is positioned by the support or spider 8. The end 15 ofthe voice coil form 14 extends above the top of the pole piece 10 todefine a chamber. To the end 15, a truncated conical diaphragm 17 isattached at its inner portion. A basket 18 is attached to the pole plate9. The outer end 19 of the diaphragm is secured to the basket 18 and aretainer 20 which is rigid in character is secured to the marginalflange or outer end 19 of the diaphragm 17. A cap 21 is adhesivelysecured to the open end of the voice coil form 14, thereby enclosing thechamber 22 defined by the inner wall of the voice coil form 14 and thetop 16 of the pole piece 10. In this chamber, a viscous, self-sustainingmaterial 23 is deposited so as to fill the chamber at its normalvolumetric capacity. This material 23 must have such a consistency thatit will not normally enter the interstitial spacebetween the inside ofthe voice coil form 14 and the outside of the pole piece 10 duringexcursions of the voice coil form 14. High viscosity oils are notconsidered too satisfactory because they tend to flow and enter into theinterstitial space. It has been found that high temperature siliconegreases, which are self-sustaining, are quite satisfactory for thispurpose. A high temperature silicone lubricant 23 is deposited in thechamber 22. It will be seen that as the voice coil form 14 moves withrespect to the top 16 of the pole piece 10, in response to the signalapplied, the volumetric capacity of the chamber 22 will change. If theexcursion of the voice coil form 14 is such as to make the chamber 22smaller, the grease or viscous material 23 will exert pressure on thecap 21 and tend to cause it to assume an externally convex position. onthe other hand, if the excursion of the voice coil 14 is such as toenlarge the capacity of the chamber'22, the grease or lubricant 23 willtend to cause the cap 21 to assume an externally concave configuration.The force required to deform the cap 21 is sufficient to dampen theexcursions of the diaphragm 17 and to produce the desired flat responsecorresponding substantially to the applied signal. The damping effectachieved is most marked in the range of 200 to 2,000 Hz., where thenatural resonance of the normal transducer frequently introducesundesirable distortions. It has been observed that the mass of viscousmaterial functions as a heat sink, and permits the application ofsubstantially greater power inputs to the voice coil of the transducer,beyond those normally applied. In spite of this, voice coil failuresfrom overheating that might normally occur due to higher input levels,do not occur under these increased load conditions. Voice coil burn-outunder higher input levels is reduced.

A speaker resonance peak of perhaps 9 to 12 decibels beyond that whichmight have been anticipated from the applied signal, in this manner isavoided.

The foregoing description is merely intended to illustrate an embodimentof the invention. The component parts have been shown and described.They each may have substitutes which may perform a substantially similarfunction; such substitutes may be known as proper substitutes for thesaid components and may have actually been known or invented before thepresent invention; these substitutes are contemplated as being withinthe scope of the appended claims, although they are not specificallycatalogued herein.

What is claimed:

-1. A damped electro-acoustic high frequency transducer comprising (a) apole piece,

(b) a voice coil form closely embracing the pole piece, and extendingbeyond it to define an open chamber with the end of the pole piece,

(c) the chamber filled with a self-sustaining viscous material,

((1) an elastomeric seal on the open end of the voice coil form toenclose the chamber and to contain the viscous mass,

(e) the volumetric capacity of the chamber varying with the movement ofthe voice coil,

(f) the seal normally tending to yield to the movement of the viscousmaterial in the chamber upon changes in the volumetric capacity of thechamber.

2. A damped electro-acoustic high frequency transducer comprising (a)the device according to claim 1, and

(b) the viscous material in the chamber being a silicone grease.

3. A damped electro-acoustic high frequency transducer comprising (a)the device according to claim 1, and

(b) a spacing spider attached to the voice coil, to position it insurrounding and spaced relation to the pole p1ece.

4. A damped electro-acoustic high frequency transducer comprising (a)the device according to claim 1, and (b) the elastomeric material havingsuflicient tensile strength to exert a damping influence on theexcursions of the voice coil. 5. A damped electro-acoustic highfrequency transducer comprising (a) the device according to claim 1, and(b) the elastomeric material having sufiicient tensile strength to exerta damping influence on the excursions of the voice coil in the frequencyrange of 200 to 2,000 Hz. 6. A damped electro-acoustic high frequencytransducer comprising (a) the device according to claim 1, and (b) theviscous material defining a heat sink, for maintaining the voice coiltemperature at a safe level, under high power input conditions.

References Cited UNITED STATES PATENTS 2,563,452 8/1951 Bozak 179-11552,655,566 10/1953 Pittinger 179--115.5 2,910,546 10/1959 Swanson179115.5 2,993,961 7/1961 Blake 179-180 X RALPH D. BLAKESLEE, PrimaryExaminer U.S. Cl. X.R.

